1. The Field of the Invention
The present invention relates generally to pigments and foils. In particular, the present invention relates to multilayered pigment flakes and foils which have magnetic layers, and pigment compositions that incorporate multilayer pigment flakes having magnetic layers.
2. The Relevant Technology
Various pigments, colorants, and foils have been developed for a wide variety of applications. For example, magnetic pigments have been developed for use in applications such as decorative cookware, creating patterned surfaces, and security devices. Similarly, color shifting pigments have been developed for such uses as cosmetics, inks, coating materials, ornaments, ceramics, automobile paints, anti-counterfeiting hot stamps, and anti-counterfeiting inks for security documents and currency.
Color shifting pigments, colorants, and foils exhibit the property of changing color upon variation of the angle of incident light, or as the viewing angle of the observer is shifted. The color-shifting properties of pigments and foils can be controlled through proper design of the optical thin films or orientation of the molecular species used to form the flake or foil coating structure. Desired effects can be achieved through the variation of parameters such as thickness of the layers forming the flakes and foils and the index of refraction of each layer. The changes in perceived color which occur for different viewing angles or angles of incident light are a result of a combination of selective absorption of the materials comprising the layers and wavelength dependent interference effects. The interference effects, which arise from the superposition of light waves that have undergone multiple reflections, are responsible for the shifts in color perceived with different angles. The reflection maxima changes in position and intensity, as the viewing angle changes, due to changing interference effects arising from light path length differences in the various layers of a material which are selectively enhanced at particular wavelengths.
Various approaches have been used to achieve such color shifting effects. For example, small multilayer flakes, typically composed of multiple layers of thin films, are dispersed throughout a medium such as paint or ink that may then be subsequently applied to the surface of an object. Such flakes may optionally be overcoated to achieve desired colors and optical effects. Another approach is to encapsulate small metallic or silicatic substrates with varying layers and then disperse the encapsulated substrates throughout a medium such as paint or ink. Additionally, foils composed of multiple layers of thin films on a substrate material have been made.
One manner of producing a multilayer thin film structure is by forming it on a flexible web material with a release layer thereon. The various layers are deposited on the web by methods well known in the art of forming thin coating structures, such as PVD, sputtering, or the like. The multilayer thin film structure is then removed from the web material as thin film color shifting flakes, which can be added to a polymeric medium such as various pigment vehicles for use as an ink or paint. In addition to the color shifting flakes, additives can be added to the inks or paints to obtain desired color shifting results.
Color shifting pigments or foils are formed from a multilayer thin film structure that includes the same basic layers. These include an absorber layer(s), a dielectric layer(s), and optionally a reflector layer, in varying layer orders. The coatings can be formed to have a symmetrical multilayer thin film structure, such as:                absorber/dielectric/reflector/dielectric/absorber; or        absorber/dielectric/absorber.Coatings can also be formed to have an asymmetrical multilayer thin film structure, such as:        absorber/dielectric/reflector.        
For example, U.S. Pat. No. 5,135,812 to Phillips et al., which is incorporated by reference herein, discloses color-shifting thin film flakes having several different configurations of layers such as transparent dielectric and semi-transparent metallic layered stacks. In U.S. Pat. No. 5,278,590 to Phillips et al., which is incorporated by reference herein, a symmetric three layer optical interference coating is disclosed which comprises first and second partially transmitting absorber layers which have essentially the same material and thickness, and a dielectric spacer layer located between the first and second absorber layers.
Color shifting platelets for use in paints are disclosed in U.S. Pat. No. 5,571,624 to Phillips et al., which is incorporated by reference herein. These platelets are formed from a symmetrical multilayer thin film structure in which a first semi-opaque layer such as chromium is formed on a substrate, with a first dielectric layer formed on the first semi-opaque layer. An opaque reflecting metal layer such as aluminum is formed on the first dielectric layer, followed by a second dielectric layer of the same material and thickness as the first dielectric layer. A second semi-opaque layer of the same material and thickness as the first semi-opaque layer is formed on the second dielectric layer.
With regard to magnetic pigments, U.S. Pat. No. 4,838,648 to Phillips et al. (hereinafter “Phillips '648”) discloses a thin film magnetic color shifting structure wherein the magnetic material can be used as the reflector or absorber layer. One disclosed magnetic material is a cobalt nickel alloy. Phillips '648 discloses flakes and foils with the following structures:                dyed superstrate/absorber/dielectric/magnetic layer/substrate;        dyed superstrate/absorber/dielectric/magnetic layer/dielectric/absorber/dyed superstrate; and        adhesive/magnetic layer/dielectric/absorber/releasable hardcoat/substrate.        
Patterned surfaces have been provided by exposing magnetic flakes to a magnetic force to effect a physical alteration in the structure of the pigment. For example, U.S. Pat. No. 6,103,361 to Batzar et al. (hereinafter “Batzar”) uses pigments made of magnetizable materials to decorate cookware. In particular, Batzar is directed toward controlling the orientation of stainless steel flakes in a fluoropolymer release coating to make patterns where at least some of the flakes are longer than the coating thickness. The patterned substrate is formed by applying magnetic force through the edges of a magnetizable die positioned under a coated base to alter the orientation of the flakes within the coating, thereby inducing an imaging effect or pattern. However, Batzar does not discuss the use of optical thin film stacks or platelets employing a magnetic layer. In addition, although the stainless steel flakes used in Batzar are suitable for decorating cookware, they are poorly reflecting.
U.S. Pat. No. 2,570,856 to Pratt et al (hereinafter “Pratt”) is directed to metallic flake pigments which are based on ferromagnetic metal platelets. Like Batzar, however, Pratt uses poorly reflecting metals and does not teach the use of thin film optical stacks.
U.S. Pat. Nos. 5,364,689 and 5,630,877 to Kashiwagi et al., (hereinafter collectively “the Kashiwagi patents”), the disclosures of which are incorporated herein by reference, disclose methods and apparatus for creating magnetically formed painted patterns. The Kashiwagi patents teach use of a magnetic paint layer, which includes non-spherical magnetic particles in a paint medium. A magnetic field with magnetic field lines in the shape of the desired pattern is applied to the paint layer. The final pattern is created by the different magnetic particle orientations in the hardened paint.
One attempt at incorporating a magnetic layer into a multilayer flake is disclosed in European Patent Publication EP 686675B1 to Schmid et al. (hereinafter “Schmid”), which describes laminar color shifting structures which include a magnetic layer between the dielectric layer and a central aluminum layer as follows:                oxide/absorber/dielectric/magnet/Al/magnet/dielectric/absorber/oxideThus, Schmid uses aluminum platelets and then coats these platelets with magnetic materials. However, the overlying magnetic material downgrades the reflective properties of the pigment because aluminum is the second brightest metal (after silver), meaning any magnetic material is less reflective. Further, Schmid starts with aluminum platelets generated from ballmilling, a method which is limited in terms of the layer smoothness that can be achieved.        
Patent Publication EP 710508A1 to Richter et al. (hereinafter “Richter”) discloses methods for providing three dimensional effects by drawing with magnetic tips. Richter describes three dimensional effects achieved by aligning magnetically active pigments in a spatially-varying magnetic field. Richter uses standard pigments (barium ferrite, strontium ferrite, samarium/cobalt, Al/Co/Ni alloys, and metal oxides made by sintering and quick quenching, none of which are composed of optical thin film stacks. Rather, the particles are of the hard magnetic type. Richter uses electromagnetic pole pieces either on top of the coating or on both sides of the coating. However, Richter uses a moving system and requires “drawing” of the image. This “drawing” takes time and is not conducive to production type processes.
U.S. Pat. No. 3,791,864 to Steingroever (hereinafter “Steingroever”) describes a method for patterning magnetic particles by orienting them with a magnetic pattern generated in an underlying prime coating that has previously been patterned by a magnetic field. The prime coat contains magnetic particles of the type MO×6Fe2O3 where M can be one or more of the elements Ba, Sr, Co, or Pb. After coating a continuous sheet of liquid coating of the primer, it is hardened and then areas of the primer are magnetized by a magnetic field. Next, a pigment vehicle with magnetic particles suspended therein is then applied. The magnetic particles suspended therein are finally oriented by the magnetic force from the magnetic pattern in the primer, creating the final pattern. However, Steingroever suffers from a diffuse magnetic image in the prime coat, which in turn passes a diffuse image to the topcoat. This reduction in resolution is because high magnetic fields are limited in the resolution they can create. This limitation is due to high magnetic field lines surrounding the intended magnetic image, thereby affecting untargeted magnetic particles in the prime coat and blurring the image.
Accordingly, there is a need for improved multilayer pigment flakes and foils with magnetic properties that overcome or avoid the above problems and limitations.